Power skiving is a denomination of a chip removing machining method, which makes use of a combination of rotary and rectilinear relative motions between a milling tool and a workpiece. Above all, the method is used in connection with the forming of teeth, splines, and the like, and allows internal, as well as external machining. The method as such is known and offers considerable advantages in comparison with traditional tooth-forming methods of the type that makes use of chip removing gear shaping and that is based on a cutting member being brought rectilinearly to and fro in order to, layer by layer, cut away material while forming a slot, which finally forms a gash of a full depth. After the shaping of the individual gash, the tool and the workpiece are readjusted stepwise so as to form, in an analogous way, all the gashes that are included in a complete gear rim. Such gear shaping is time-consuming and thereby expensive, in particular when the number of teeth is large.
In contrast to gear shaping, power skiving only requires a minimum of tool readjustments in that the tool during a planetary relative motion in relation to the workpiece is kept in constant rotation in order to remove, from each gash to be made, only one fragmentary chip, at the same time as the tool is given a rectilinear axial feeding motion. In such a way, all gashes are machined to a certain, per se moderate, depth before the tool needs to be readjusted for providing an increased radial feeding.
The earliest developed power skiving tools (see, e.g., EP 2422920, WO2010102992, and EP 2440357) make use of solid milling cutter heads, of cemented carbide, i.e., bodies in which the requisite cutting edges are integrated and therefore can be given a great spatial dimensional accuracy in relation to the rotation axis of the tool.
However, a disadvantage of solid power skiving tools is that they can successfully only be realized in small embodiments, more precisely with diameters of up to 150 mm and a maximum of approximately 10 cutting edges. For the machining of large workpieces, e.g. gear rings having more than 100 teeth, greater tools are desired, i.e., tools having a greater diameter and more cutting edges.
Another disadvantage is that the tool has to be discarded (or be subjected to extensive repair work) if any single cutting edge is damaged. For these reasons, a development of power skiving tools having replaceable cutting inserts, most often more than 10 in number, has recently started.
An initially mentioned power skiving tool, having as many replaceable cutting inserts as 20, is previously known by open use (manufactured by VBMT). In this case, the cutting inserts have a rhombic basic shape and include two diametrically opposed and alternately usable cutting edges of identical shape. The cutting insert is mountable in a seat, which, in addition to a flat bottom, includes two V-shaped diverging side support surfaces, and fixable by means of a tightening screw, which is centrally arranged in the cutting insert and presses the cutting insert against the bottom, as well as the two side support surfaces in the seat.
A problem of this known power skiving tool is, however, that good accuracy of the radial adjustment of the cutting inserts in relation to the basic body of the tool is difficult to realize. Namely, if the individual cutting insert and/or the receiving seat in the basic body would be impaired by form defects, the active, radially outer cutting edges will not be located exactly along a common circle. This result in defects in the flank surfaces of the formed teeth, more precisely in such a way that repetitive slots arise in/of the tooth flanks, if one or a few cutting inserts would protrude further than other ones. On the other hand, it applies that repetitive crests in the flanks arise if one or a few cutting edges would be located further in than other ones. Therefore, the desired surface quality of the tooth flanks requires a setting accuracy or tolerance in the order of 0.001-0.002 mm, something that is practically difficult, not to say impossible, to achieve in the known power skiving tool.
Another disadvantage of this is that the cutting inserts become fairly blunt-cutting, something that may lead to a strong generation of heat and that the rotating workpiece together with the tool has to be driven at a limited number of revolutions. A shortcoming of the known tool is furthermore that the cutting inserts, by their rhombic and mirror-symmetrical shape, may present difficulties to produce deep gashes and teeth, respectively, having a more complicated shape, such as involute teeth.
By SE 0103951-0 (publication number SE523286), a milling tool is previously known in the form of a face mill or end mill having a basic body, which is equipped with tangentially spaced-apart cutting inserts, which are mounted in each a seat in the axially rear end of which an adjusting mechanism is arranged for the fine adjustment of the axial position of the individual cutting insert in relation to the basic body. In this case, the cutting inserts are indexable by including two opposite and alternately usable cutting edges adjacent to the same number of clearance surfaces. When one, face milling cutting edge is indexed up into an active position, the clearance surface of the opposite cutting edge is kept pressed against a movable jaw included in the adjusting mechanism. This entails a risk of the fine adjustment of the cutting insert being jeopardized, namely if the used cutting edge—and its clearance surface—which is turned rearward toward the adjusting mechanism, has been damaged, e.g. by chipping or plastic material deformation as a consequence of strong generation of heat. Namely, if possible damage propagates to the clearance surface, even such moderate deformations as approx. 0.1 mm will make impossible predeterminable fine adjusting in the range down to 0.001 mm. By forming, in accordance with the invention, the cutting insert with only one, front cutting edge at the same time as the rear end of the cutting insert consists of a shoulder surface interacting with the adjusting mechanism, the above-mentioned risk is obviated.